1. Field of the Invention
The present invention relates to a supervisory control method and a power saving control unit for a computer system and, more particularly, to a method for detecting, with high probability, the status of the computer system wherein the CPU is waiting for the next substantial task to start while repeatedly executing a small loop program (which status will hereinafter be referred to as a substantial rest state) and a power saving control unit for reducing the power consumption of the CPU in the substantial rest status through utilization of the supervisory control method.
2. Description of the Prior Art
As disclosed in Japanese Patent Laid-Open Publication No. 178818/90, for example, it is well-known in the art to stop power supply to those sections of a computer system which are not executing any substantial tasks so as to reduce the total power consumption, and this technology has been put to practical use in various forms. In the field of battery-driven, portable personal computers including a lap-top computer, in particular, much study is now being given power saving technology of this kind with a view to maximize the system uptime with a smaller and lighter battery.
A certain known personal computer possesses two kinds of standby functions commonly referred to as a rest mode and a sleep mode. The rest mode is a function which automatically switches the clock frequency from 16 MHz to 1 MHz when the CPU does not operate for a predetermined period of time. A certain elapsed time thereafter the computer automatically enters the sleep mode, in which the power supply is stopped. Whichever mode the computer is operating in, it will return to a normal mode upon pressing an arbitrary key. The period of time for which the CPU does not operate before the computer goes into the standby mode can arbitrarily be set by individual users.
Thus, it is a condition for the computer to enter the reduced power consumption status, i.e., the above-mentioned standby mode, that the CPU does not operate for any substantial task for a predetermined period of time. More specifically, the computer assumes the reduced power consumption status when an external factor which starts the CPU for a substantial task, such as an input from the keyboard or communication controller, does not occur for a predetermined period of time.
In the prior art in which the CPU is regarded as being in the substantial rest status when the above-mentioned external factor does not occur for a predetermined period of time, power consumption cannot sufficiently be reduced, because it is necessary to set the "predetermined period of time" to more than tens of seconds for ordinary personal computers.
Now, let it be assumed that a personal computer is operating under Japanese word processing software. In this instance, every input from the keyboard constitutes a factor which activates the CPU. In response to each input signal the CPU performs a very simple task of displaying a character on a display, a little complicated task such as a kana character to kanji character conversion or moving a document, or a little more complicated and hence time-consuming task such as adjustment of files. When an operator keys while elaborating sentence, the processing speed of the CPU is generally far higher than the keying speed of the operator, and in most cases a substantial rest period as short as tens of milliseconds to several seconds occurs between key inputs.
If the aforementioned "predetermined period of time" is set to one to several seconds taking such a substantial rest time into account, then the CPU will go into the power saving mode prematurely before it completes processing in the case of performing a little time-consuming task such as moving a document or adjusting a file. It is therefore necessary to set the "predetermined period of time" to tens of seconds to several minutes, allowing for a sufficient margin for safety. This, however, keeps the power saving function from serving during such short-time substantial rest periods which occur frequently, hence power dissipation cannot sufficiently be reduced.
One possible solution to this problem is to construct the computer system so that each time the CPU enters the substantial rest mode it indicates to an external power saving control circuit that the power saving mode is applicable. Yet this calls for incorporating some additional functions of features into software under which the CPU operates. It is very cumbersome to add such new functions to the existing software. It is a condition of the present invention that no modifications be introduced into the existing software, and the possible solution does not meet this condition.
Furthermore, if the computer system works with only one piece of software, it is possible to pre-analyze the contents of the software to thereby accurately detect the substantial rest state of the CPU wherein the CPU repeatedly executes a small loop program while accessing only a particular address group. It is impractical, however, to take this approach for various pieces of software that the computer system executes.